HSP70 sequestration by free α-globin promotes ineffective erythropoiesis in β-thalassaemia

Abstract

β-Thalassaemia major (β-TM) is an inherited haemoglobinopathy caused by a quantitative defect in the synthesis of β-globin chains of haemoglobin, leading to the accumulation of free α-globin chains that form toxic aggregates. Despite extensive knowledge of the molecular defects causing β-TM, little is known of the mechanisms responsible for the ineffective erythropoiesis observed in the condition, which is characterized by accelerated erythroid differentiation, maturation arrest and apoptosis at the polychromatophilic stage. We have previously demonstrated that normal human erythroid maturation requires a transient activation of caspase-3 at the later stages of maturation. Although erythroid transcription factor GATA-1, the master transcriptional factor of erythropoiesis, is a caspase-3 target, it is not cleaved during erythroid differentiation. We have shown that, in human erythroblasts, the chaperone heat shock protein70 (HSP70) is constitutively expressed and, at later stages of maturation, translocates into the nucleus and protects GATA-1 from caspase-3 cleavage. The primary role of this ubiquitous chaperone is to participate in the refolding of proteins denatured by cytoplasmic stress, thus preventing their aggregation. Here we show in vitro that during the maturation of human β-TM erythroblasts, HSP70 interacts directly with free α-globin chains. As a consequence, HSP70 is sequestrated in the cytoplasm and GATA-1 is no longer protected, resulting in end-stage maturation arrest and apoptosis. Transduction of a nuclear-targeted HSP70 mutant or a caspase-3-uncleavable GATA-1 mutant restores terminal maturation of β-TM erythroblasts, which may provide a rationale for new targeted therapies of β-TM.

Unexpected macrophage-independent dyserythropoiesis in Gaucher disease

“…However, the dyserythropoiesis in this disease is characterized not only by accelerated erythroid differentiation, but also by maturation arrest and apoptosis at the polychromatophilic stage. [34][35][36] In our study of GD subjects, the increased erythroid differentiation was associated with a decrease in cell expansion, but we did not observe any premature death of the erythroid precursors or maturation arrest. This may explain the mild anemia in GD compared to the much more severe anemia observed in patients afflicted with β-thalassemia.…”

Thalassemia 2016: Modern medicine battles an ancient disease

“…This results in maturation arrest and apoptosis [80]. Genetic manipulations such as transfecting an uncleavable GATA-1 or an HSP70 targeted to the nucleus, restored normal maturation of thalassemia erythroblasts [80]. These findings may be applicable to the development of novel therapies directed at free a-globin chains or HSP70.…”

“…However, in b-thalassemia, HSP70 interacts with free a-globin chains in the cytoplasm, which sequester it there and prevent it from translocating to the nucleus to protect GATA-1. This results in maturation arrest and apoptosis [80]. Genetic manipulations such as transfecting an uncleavable GATA-1 or an HSP70 targeted to the nucleus, restored normal maturation of thalassemia erythroblasts [80].…”

Pomalidomide reverses γ-globin silencing through the transcriptional reprogramming of adult hematopoietic progenitors

“…2 Therefore, b-hemoglobinopathies have been the focus of numerous studies aimed at elucidating the molecular pathophysiology of these diseases and developing new therapeutic strategies. 3 Although the pathophysiology of b-thalassemia and SCD differ, [4][5][6][7][8][9] clinical symptoms do not manifest before birth because b-globin chains are expressed postnatally. Because reactivation of fetal hemoglobin (HbF) ameliorates anemia and associated complications of SCD, 10 strategies for HbF induction have long been pursued as therapeutic options.…”